1. Field of the Invention
The present invention relates to the preparation of alkaline earth metal titanates, and, more especially, to the preparation of alkaline earth metal titanates directly in the form of homogeneous powders by chemical reaction in molten salt baths.
2. Description of the Prior Art
The alkaline earth metal titanates, and, in particular, barium titanate, are known to this art to have preferred applications in the preparation of ceramic compositions, notably intended for the production of electric capacitors and resistors. Different methods for the preparation of barium titanate are also known to this art. In particular, such compound may be prepared by solid-state reaction between a barium salt including a volatile anion, for example barium carbonate, and titanium dioxide.
However, this method has the disadvantage of delicate controls, as it is based on slow and difficult diffusion reactions between crystallized solids, requires very high temperatures and often presents the risks, on the one hand, of formation of heterogeneous solid solutions, and, on the other, the introduction of detrimental impurities during the subsequent grinding stage, which generally must be carried out after calcination.
More recently, it was discovered that it is also possible to prepare the subject titanates via the calcination or thermal decomposition of precursor compounds produced by various reactions taking place in liquid phase, typically aqueous phase. The major disadvantage of this method resides in the difficult control of the quality of the precursors obtained, such that the pyrolysis of the latter may result in the production of barium titanate particles, the composition and/or the particle size distribution of which are relatively heterogenous.
A third and more favorable method is described in U.S. Pat. No. 4,293,534, which method is based on direct chemical reactions conducted in molten salt baths of the hydroxide type, according to the following general reaction scheme: EQU BaO+TiO.sub.2 .fwdarw.BaTiO.sub.3
or EQU SrO+TiO.sub.2 .fwdarw.SrTiO.sub.3
From the different examples presented in this '534 patent, it will be seen that the industrial application of the process described requires at least four distinct and successive stages, which are:
(1) The suspension, under vigorous agitation in water, of a mixture of TiO.sub.2 and Sr(NO.sub.3).sub.2 in proportions corresponding to the stoichiometry of the final product;
(2) The addition to such suspension of a highly concentrated solution of sodium hydroxide and potassium hydroxide (solution near saturation), having the effect of causing the formation of a gel; over the course of this operation Sr(NO.sub.3).sub.2 is converted into Sr(OH).sub.2 ;
(3) Heating the gel thus obtained at a temperature on the order of 200.degree. to eliminate the water and effecting the conversion of Sr(OH).sub.2 into SrO;
(4) Calcination at elevated temperatures (600.degree.) such that, in a bath of molten hydroxides, the following reaction is carried out: EQU SrO+TiO.sub.2 .fwdarw.SrTiO.sub.3
It follows from the above analysis that the process described in the '534 patent is difficult to carry out (crucibles must be changed between stage 3 and stage 4), is of long duration (total duration of more than 15 hours) and is quite expensive relative to energy requirements (evaporation of a large amount of water, relatively high calcination temperatures) and that, in its final stage, the process reverts to reacting a titanium oxide and a barium or strontium oxide in a more than 40% by weight molten hydroxide bath.
Furthermore, the quality of the final products may be inadequate.